This invention relates to the purification of waters containing hexavalent chromium compounds. In particular, the invention relates to a process for chemically reducing or eliminating toxic hexavalent chromium compounds from industrial process effluent or rinse waters such as waters resulting from chromate oxidation or chrome electroplating processes.
A significant pollutant found in industrial waste waters is the highly toxic hexavalent chromium, most often in the form of chromates, bichromates or chromic acid used in oxidation processes, as in chrome dyeing and tanning. An additonal source of hexavalent chromium compounds is the effluent from chrome electroplating and in the anodizing of aluminum, such as rinse waters used in or resulting from such processes. Regulatory agencies in many states now require that industrial effluents contain 1 ppm. or less of hexavalent chromium.
While it is known that these highly toxic substances are removed by reduction with sulfur dioxide or hydrazine to form relatively non-toxic trivalent chromium compounds, the former in connection with sulphite-bisulphite systems, such processes are cumbersome, inefficient, difficult to control, and present health hazards due to the presence of the sulphur dioxide or hydrazine.
Processes are also known in which chromium compounds are recovered from spent caustic chromium ore baths by reduction with elemental iron. Such techniques have remained slow and inefficient, even when reduction to the trivalent state was not required.